1. Field of the Invention
The present invention relates to a light source device, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
An optical scanning device used for an image forming apparatus such as an optical printer, a digital copier, and an optical plotter scans a surface to be scanned with light modulated according to image information and forms a latent image according to the image information on the surface. The optical scanning device drives a light source using a modulated signal pulse-modulated according to image information in order to output light modulated according to the image information.
Typically, a semiconductor laser is used as the light source, and an edge emitting semiconductor laser (hereinafter, also referred to as an “edge emitting laser”) that outputs light in a direction parallel to a substrate has been dominant. However, in recent years, a vertical cavity surface emitting laser (VCSEL) comes on the market. The VCSEL has characteristics of (1) low costs, (2) low electrical power consumption, (3) small size and high performance, and (4) easy two-dimensional integration as compared with the edge emitting laser.
Semiconductor lasers have characteristics such as droop characteristics, rising characteristics, and falling characteristics, in which the light amount changes over time. Such characteristics are known to be derived from the change of threshold current due to heat of an element itself generated by applying current, and from the CR time constant of an electric circuit. These characteristics may cause difference in the image density or may cause an image defect such as density unevenness and color unevenness. For example, Japanese Patent Application Laid-open No. 2006-91157, Japanese Patent Application Laid-open No. 2005-156933, and Japanese Patent Application Laid-open No. 2006-259098 disclose a method for controlling the characteristics. In the method, an optical scanning device receives, by a detector such as a photodiode, a part of a light beam output from a light source as a monitoring light beam and performs auto power control (APC) for controlling the output level of the light source based on the received result.
Image forming apparatuses require higher image density in order to improve image quality and higher output speed of images in order to improve operability. For example, Japanese Patent Application Laid-open No. 2003-283031 discloses a method for achieving both the higher image density and the higher output speed. In the method, an optical scanning device including a light source having a plurality of light-emitting elements scans a surface to be scanned with a plurality of light beams at a time.
Moreover, for example, Japanese Patent Application Laid-open No. 2006-332142 and Japanese Patent Application Laid-open No. 2008-213246 disclose various measures for solving problems arising from the use of a plurality of light-emitting elements.
In a semiconductor laser, a drastic temperature change in the active layer caused by supplying driving current causes a change in refractive index and thus changes the optical confinement state. Therefore, the divergence angle of a light beam to be output (far field pattern (FFP)) is small immediately after current application and becomes large over time even when the driving current is kept constant.
In an optical scanning device including an optical system in which a light beam is collimated with an aperture member, such change in the divergence angle changes the light amount on the surface to be scanned.
An edge emitting laser typically causes mode hopping (wavelength hopping) during driving for quite a short time. Therefore, when the length of an optical path of a cavity is changed due to heat, or when the gain function of a laser medium is changed due to drastic characteristic fluctuation caused immediately after the application of driving current, a mode jump may occur toward a mode most advantageous to oscillation, that is, a mode with a large gain.
For example, as illustrated in FIG. 25, a mode at a short wavelength side (648.17 nanometers) rises immediately after the application of driving current, and a mode at a long wavelength side becomes predominant in stages (mode hopping), and ultimately, the modes are stabilized to become one mode. In FIG. 25, the interval between the adjacent modes is 0.16 nanometer. In a typical edge emitting laser having a wavelength of 650 nanometers, the interval between the adjacent modes is quite small, e.g., about 0.2 nanometer, and thus, the optical system is hardly affected. In other words, even when the inner state of an edge emitting laser is changed, its optical output is relatively stable.
On the other hand, the cavity length of a VCSEL is only about one wavelength, and therefore, mode hopping does not occur in theory. This is because the wavelengths of the adjacent modes are positioned far apart from the oscillation wavelength and are, for example, half or twice the oscillation wavelength. For example, in a VCSEL having an oscillation wavelength of 780 nanometers, the adjacent mode is 390 nanometers or 1560 nanometers. Accordingly, in the VCSEL, even when its inner state is changed, oscillation is continued in the same mode, and thus, the optical output is changed depending on the change in the inner state. In other words, the change in the inner state changes the light amount on the surface to be scanned.
In recent years, image forming apparatuses are used also for simple printing as an on-demand printing system, and in accordance with this, image forming apparatuses further excellent in image quality are required.
However, it seems to be difficult for conventional electrical driving control methods of a light source for controlling the light amount change on a surface to be scanned to correspond to further improvement of image quality demanded in the future.
Typical optical scanning devices and image forming apparatuses are required to have light sources whose optical output ranges are wide to some extent because of the following reasons.
Reason 1: The fluctuation occurs in light use efficiency due to production errors of optical elements.
Reason 2: Production errors of photosensitive elements, toner, developing agents, and similar elements occur.
Reason 3: Image density needs to be adjusted according to environmental changes and aged deterioration.
When the optical output range of the light sources is narrow, the following disadvantages occur.
Disadvantage 1: Costs increase because the production accuracy of optical elements needs to be improved.
Disadvantage 2: Costs increase because a screening process is required in order to decrease the fluctuation of photosensitive elements, toner, developing agents, and similar elements.
Disadvantage 3: Image quality deteriorates because image density cannot be adjusted sufficiently.
Among the three disadvantages, the disadvantage 3 cannot be overcome even if the cost increase is accepted, and therefore, the only measure for obtaining images with high quality is to ensure a wide optical output range.
However, when a VCSEL is used within a wide optical output range, fluctuation due to the unstable state of the light amount at start-up may occur. This fluctuation is a peculiar phenomenon to VCSELs.
Image density adjustment is described below.
In an image forming apparatus such as a copier and a laser beam printer that employ electrophotography, image density control for adjusting image density so as to always obtain intended image density is in practical use. The control is performed at a predetermined timing (when power is tuned on, every predetermined time, or every predetermined numbers of sheets) by exposing a photosensitive element to light while latent image electrostatic potential is changed, forming a latent image on the photosensitive element, and detecting toner density of a toner image obtained by visualizing the latent image with toner by an optical density sensor.
The typical image density control detects a change in characteristics of latent image electrostatic potential relative to the exposed amount of a photosensitive element, feeds back the detected result, and sets optimal charge potential and optical output from a light source.
Disadvantages when image density control is performed while optical output is kept constant are described below.
FIG. 26A illustrates a halftone characteristic prior to adjustment. Development potential (charging-developing bias) is adjusted in order to obtain desired solid density (density when writing duty is 100 percent). Typically, pulse width modulation (PWM) is used for obtaining halftone density.
FIG. 26B illustrates a halftone characteristic after development potential is adjusted. Although the solid density can be adjusted, the halftone density has not yet reached intended density, and halftone reproducibility is low.
FIG. 26C illustrates a halftone characteristic after laser diode (LD) power control is performed on the image of FIG. 26B. An intended halftone characteristic can be obtained.
In other words, when the optical output range of a light source cannot be adjusted, the adjustment is performed only to the level of the image in FIG. 26B. As a result, a halftone characteristic deteriorates, thereby deteriorating image quality.